Ex. 4. W. E. 1887, Sept. 23rd, at 6h om A.M., apparent time at ship, latitude 24° 40′ S., longitude 45° 31′ W., sun's magnetic amplitude W. 2° 50′ N., variation 7° 42′ E. Ex. 5. 1887, December 9th, at 8h 27m A.M., apparent time at ship, latitude 54° 35′ N., longitude 53° 15′ W., sun's magnetic amplitude S.E. S.E.E. E., variation 36° 20′ W., ship's head Decl. at noon, Dec. 9th, 22° 49′ 46′′ S. The Green. date being noon, Dec. 9th, one of the instants for which the decl. is put down in the Almanac, nothing more is necessary thin to transcribe the quantity as there put down. sine 9.588890 54 35 Ex. 6. 1887, July 1st, at 8h 36m P.M., apparent time at ship, latitude 56° 4' N., longitude 64° 50' W., sun's setting amplitude North, variation 30° 0' W. Ship date (A.T.), July 1d 8h 36m os Decl., page I, N.A., July 1st, at noon, 23° 7' 36" 6 N., decr.; var. 19" 98; then 9'98 X 12h 92 = 128" 9416, or 2 89, the correction, which subtracted from decl. at noon, 23° 7′ 37 N., gives Red. decl. 23° 5' 28 N. sine 9'593511 sine 9.846699 Ex. 7. 1887, June 19th, at 9h 40m P.M., apparent time at ship, lat. 62° 31' N., long. 30° 24′ W., sun's setting amplitude N.N.E.; and supposing the variation of the compass is 57° 50' W., required the deviation for the position of the ship's head at the time the observation was taken. Or, the error of compass being subtracted from the variation, the deviation is of the same name as the error. Otherwise:-Having found the true amplitude, which is reckoned from W. to N., we may add 90° to the number of degrees the observed bearing is East of North, whence we have the observed amplitude (22° 30′ + 90) = W. 112° 30' N. The true and observed amplitudes must both be reckoned from the same point of the compass, E. or W., but in this instance one is reckoned from W. and the other from E.; therefore, by taking either of them from 180°, they would both be reckoned from the same point -the true amplitude, in this example, is taken from 180°, and it is then reckoned from E. instead of W. Next take the difference of the amplitudes, as they are both marked N.; and since the true amplitude is to the left of the magnetic-looking from the centre of the compass in the direction of the magnetic-the error of compass is W. The error of compass and variation being of the same name, take their difference for the deviation, which mark E., because the error of the compass is to the right of variation, looking from the centre of the compass in the direction of the variation. EXAMPLES FOR PRACTICE. In each of the following examples the Error of Compass and Deviation are required for the position of the ship's head at the time of observation. "BY THE ADMIRALTY TIDE TABLES.” 159. These Tide Tables, published annually, give the time (A.M. and P.M.) of high water, and the height for every day in the year, at the following places, viz. :-Brest, Devonport, Portsmouth, Dover, Sheerness, London, Harwich, Hull, Sunderland, North Shields, Leith, Thurso, Greenock, Liverpool, Pembroke, Weston-super-mare, Holyhead, Kingston, Belfast, Londonderry, Sligo Bay, Galway, Queenstown, and Waterford. 160. To find the times of high water from the Tide Tables if the place is one of the Standard Ports, proceed by RULE LXVI. Turn to the month in the Tide Tables and find the given place; then opposite the given date will stand the morning (A.M.) and afternoon (P.M.) times of high water required. NOTE. When the mark — occurs it shows that there is but one tide during that day; no high water, therefore, takes place in the morning or afternoon in which the mark appears. Thus, wishing to know the time of high water at North Shields on the 1st of February, 1885-on turning to February under the head of North Shields (see page 13), it is seen at a glance that high water takes place at 4h 20m A.M., and that the height of tide is 14 ft. 2 in. above the mean low water level of spring tides, and that the time of high water on the afternoon of same day is 4b 43m, while the height of tide above the low water level of spring tides is -- 14 ft. 2 in. Similarly, desiring to know the particulars of the tide at Brest on the morning of April 10th, 1885 (see page 26), the mark shows that no tide occurs on the morning of that day; there will be a high water at 11h 41m P.M. on the 9th, and again at oh 18m P.M. (i.e., 18m past noon of the 10th April, but none in the interval. Again, if it be required to know the times of high water on May 1st, 1885, at Westonsuper-mare-on turning to May, and under Weston-super-mare (see page 39), and opposite the 1st wo find that the times of high water are 8h om A.M., and 8h 16m P.M. respectively. 161. If the place at which the time of high water is required be not a Standard Port, it is to be referred (if in the west of Europe) to a Standard Port, by adding or subtracting a certain constant to the time of that Standard Port, as directed in the Tables. In pages 103 to 108 of the Admiralty Tables, 1885, will be found upwards of two hundred ports on the coasts of the United Kingdom, and in Europe, for which Standard Ports of Reference are given, and the time which is to be added to or subtracted from the time of high water at such Standard Port. 162. To find the times of high water by the Tidal Constants. RULE LXVII. 1o. Seek in the "Tide Tables," pages 104-108, in the left-hand column for the given place, or port you wish the time of high water for, and in the column headed "Standard Port for Reference," will be found the Standard Port for the given place; also, from the middle column headed "Time," and opposite the given place, take out the "Constant," being careful to note whether it is additive (marked +), or subtractive (marked —). 2°. Take out of "Admiralty Tide Tables," pages 1-97, the morning (A.M.) and afternoon (P.M.) times of high water at the "Standard Port for Reference," being careful to annex the letters A.M. or P.M. to the tides so taken out. (a) If a blank (—) occurs in either morning (A.M.) or afternoon (P.M.) column, use the preceding time of high water instead when the Constant is marked additive (+), but use the time of high water following the blunk (· when the difference is marked subtractive (—). 3°. To the times of high water at the Standard Port just taken out, apply the Constant (No. 2°), adding or subtracting said Constant according as it is marked +or; the result in each case, if less than 12", is respectively the morning (A.M.) and afternoon (P.M.) times of high water required. (See Exs. 1 and 2). (a) When the sum of the Constant and the morning (A.M.) time of high water at the Standard Port exceeds 12", deduct 12", the remainder is the afternoon (P.M.) time of high water at the given place. To obtain the morning (A.M.) time of high water at the given place, if any, add the Constant to the preceding afternoon (P.M) time of high water at the Standard Port, and if the sum exceeds 12", deduct 12", the remainder is the morning (A.M.) tide sought (Ex. 3), but if the sum be less than 12", it is the afternoon (P.M.) tide of the day before, and there is no morning (A.M.) tide that day at the given place. (Ex. 4). (b) When the Constant added to the morning (A.M.) time of high water at the Standard Port is less than 12" (i.e., gives morning (A.M.) tide at given place), but when added to the afternoon (P.M.) tide at the Standard Port is greater than 12, there is only a morning (a м.) tide at the given place on that day. (Ex. 5). NOTE.-When the sum of the Constant and the tide taken from the Tables is less than 12h, it remains a tide of the same name as that used, but when the sum exceeds 12h, the time over 12b will be a tide of the name following that taken out; consequently, in such a case take from the Tables the tide immediately preceding the one you require. (c) When the Constant is subtractive, and exceeds the morning (A.M.) time of high water at the Standard Port, reject this last and use the following afternoon (P.M.) tide at the Standard Port (Ex. 10). If the subtractive Constant exceeds the afternoon (P.M.) tide at Standard Port, 12" must be added to this last before subtraction is made, the remainder will be the morning (A.M.) tide at the given place. For the afternoon (P.M.) tide use the following tide at Standard Port, that is, the morning (A.M.) time of high water next day, borrowing 12h if Constant exceeds it, the remainder is afternoon (P.M.) tide at the given place (Ex. 10). (d) If Constant being subtractive, exceeds the Standard Port morning (A.M.) high water, but is less than the Standard afternoon (P.M.) tide, there is only an afternoon (P.M.) tide at the given place on that day (Ex. 12). (e) If when the Constant is subtractive, the Standard Port afternoon (P.M.) tide has to be increased 12, but Constant is less than the Standard morning (A.M.) tide following; there is only a morning (A.M.) tide at the given place that day. Short Rule for Blank.-First apply the Constant to the tide given; then, if Constant is + borrow last night's tide, but if -- borrow next morning's tide, being careful to use the given tide first. In either of the above cases should the tide prove useless, then write down no A.M. or no P.M., according to the result of the given tide. EXAMPLES. Ex. 1. 1885, January 16th: find the times of high water at Scarborough. Turning to Admiralty Tide Tables for 1885, at page 106, in the left-hand column, and under the head of "Ports of Great Britain," we find Scarborough, and in the right-hand column, immediately abreast, we find that the Standard Port for Reference in this instance is Sunderland, and in the column under Time we have the Constant + ob 49; that is, we have to add oh 49" to the time of high water at Sunderland on any day in order to obtain the corresponding time of high water at Scarborough. The work will stand as follows:Port for Reference-Sunderland, Jan. 16th 3h 25m A.M. Constant for Scarborough Time of H.W. Scarborough, Jan. 16th +49 4 14 A.M. Jan. 16th 3h 42m P.M. +49 Jan. 16th 4 31 P.M. Ex. 2. 1885, January 7th: find the times of high water A.M. and P.M. at the Needles Point. Turning to the "Admiralty Tide Table" for 1885, at page 107, in the left-hand column, we find Needles Point, and in the right-hand column, immediately abreast, we find that the Standard Port for Reference, which in this instance is Portsmouth, and in the column under Time we have the Constant - 1h 55m; that is, we have to subtract 1h 55m from the time of high water at Portsmouth on any day in order to obtain the corresponding time of high water at Needles Point. The work will stand as follows:Port for Reference-Portsmouth, Jan. 7th 342m A.M. Constant for Needles Times H.W. Needles, Jan. 7th Jan. 7th 4h 4m P.M. - I 55 |